/*
// Definition for a Node.
class Node {
public:
    int val;
    Node* left;
    Node* right;
    Node* next;

    Node() : val(0), left(NULL), right(NULL), next(NULL) {}

    Node(int _val) : val(_val), left(NULL), right(NULL), next(NULL) {}

    Node(int _val, Node* _left, Node* _right, Node* _next)
        : val(_val), left(_left), right(_right), next(_next) {}
};
*/

class Solution {
public:
    Node* connect(Node* root) {
        if(root == nullptr) return nullptr;
    queue<Node*>q;
    q.push(root);
    while(!q.empty())
    {
        int n = q.size();
        Node*last =  nullptr;
        for(int i = 1;i<=n;++i)
        {
            Node*f = q.front();
            q.pop();
            if(f->left)
            {
                q.push(f->left);
            }
            if(f->right)
            {
                q.push(f->right);
            }
            if(i!=1)
            {
                last->next = f;
            }
            last = f;
        }
    }
    return root;
    }
};
